The lungs can be characterized as a mass exchanger in which oxygen, anesthetics and/or medication are delivered through the alveoli to blood pumped from the heart, and carbon dioxide, and anesthetics during emergence, are removed from the blood for exhalation. The mass transfer rate and efficiency in either direction, i.e. removal or inflow of gaseous materials at the blood/gas interface, is dependent at least in part on the distribution of ventilation to each lung. In turn, mechanical factors such as compliance and flow resistance within the bronchi and the different regions of the lungs affect the distribution of pulmonary ventilation. The term "compliance" refers to the elasticity of the lungs, or their ability to expand and contract during an inspiration and exhalation cycle, and is the inverse or mathematical reciprocal of stiffness. The flow resistance along the respiratory pathways refers to blockages or restrictions to the passage or flow of gaseous materials to and from the lungs.
Diseased or injured lungs may have markedly different compliances or flow resistances compared to healthy lungs. For example, one bronchus may have a higher flow resistance due to swelling of its mucus membrane that constricts its flow area compared to the bronchus associated with the other, unaffected lung. Additionally, one lung could be less compliant than the other due to trauma or aspiration of gastric acid from the stomach. A lung with lower resistance and/or lower compliance builds up pressure at a faster rate than the other lung when both are exposed to a common pressure or flowrate input at the trachea, or at the carina where the bronchi meet the trachea. Consequently, the distribution of ventilation in the lungs can become unequal such that the volume of gas in the right lung at the end of inspiration may not be equal to the volume of gas in the left lung. If both an abnormal lung and a healthy lung receive similar blood perfusion rates (Q), i.e. the same volume of blood from the heart per unit time (cardiac output), but different ventilation or gas volume rates (V), there is an undesirable ventilation/perfusion ratio (V/Q) mismatch. This mismatch degrades the mass transfer or gas exchange rate, and the efficiency of such exchange, within the lungs. In turn, less carbon dioxide (and less gaseous and volatile anesthetics during emergence) come out of solution from the blood, while less oxygen (and less gaseous and volatile anesthetics during induction and maintenance) dissolve into the blood per unit time. Although the body's compensatory mechanisms will shunt the perfusion to favor the better ventilated lung, there is a limit to that self-regulatory action, particularly when it is depressed by some anesthetics.
Another physiological parameter which is of concern during ventilation and/or anesthesia of a patient is the mean pressure within the lungs over time (mean lung pressure, MLP). Higher mean lung pressures during mechanical ventilation can reduce the cardiac output or volume of blood pumped by the heart per unit time by interfering with the filling and emptying of the heart. Because the lungs and heart both reside in the chest cavity, excess pressure, and hence excess expansion of the lungs, can reduce cardiac output.
In view of these problems with ventilation of diseased or injured lungs, one design objective of ventilation apparatus and anesthesia systems is to equalize the distribution of ventilation in lungs of unequal compliances and/or unequal resistances, while minimizing the mean lung pressure within the lungs. We numerically express distribution of ventilation as the "ventilation distribution ratio," or quotient of the volume of gas within the right lung over the volume in the left lung at the end of inspiration. Assuming the right and left lung to be of equal volume, the ventilation distribution ratio (R.sub.v) should ideally be unity, or, for diseased or damaged lungs, as close to unity as possible.
A number of studies have been undertaken to determine the effectiveness of inspiratory waveform shaping as a means of optimizing the ventilation distribution ratio of lungs having unequal compliance and/or resistance, while minimizing mean lung pressure. The term "inspiratory waveform shaping" refers to the configuration of a pressure or flowrate waveform over time which is delivered by a ventilator (ICU (intensive care unit) or anesthesia) to the patient during mechanical inspiration. Currently, there are four flowrate waveforms commonly employed in ICU ventilators, including constant flowrate, linearly increasing flowrate, linearly decreasing flowrate and half-sine (0 to .pi.) flowrate. Most anesthesia ventilators offer only a constant flowrate waveform. These flowrate waveforms have been utilized in various studies to assess the effect of using one waveform or another on the ventilation distribution ratio for different lung configurations, e.g. lungs having equal compliance and unequal resistance (ECUR), and lungs with unequal compliance and equal resistance (UCER).
The results obtained from prior studies involving inspiratory waveform shaping have up to now contradicted one another. Furthermore, none of the studies have addressed the need for a lung classification scheme or how to identify/classify a patient's lung configuration. No suggestion is made as to how one might determine the particular characteristics of the lung of a given patient so that an appropriate inspiratory waveform or other ventilatory parameters might be selected and utilized. Further, the emphasis in prior studies has been to attempt to determine the "best" single pressure or flowrate inspiratory waveform for all types of lung conditions, even though the characteristics and behavior of lungs afflicted with emphysema, asthma and acute respiratory distress syndrome, for example, are very different.